In recent years, an electronic endoscope having the function as a spectrometer described, for example, in Japanese Patent Provisional Publication No. JP2007-135989A has been proposed. Through the electronic endoscope of this type, a spectral property (distribution of an optical absorption factor with respect to frequency) of a biological tissue, such as a mucous membrane of a digestive organ, e.g., a stomach or an intestinum rectum can be obtained. The spectral property depends on types of substances included in a surface layer of the biological tissue target for measurement of the spectral property (Beer-Lambert Law). That is, the spectral property of a biological tissue is obtained by combining the spectral properties of a plurality of primary substances forming the biological tissue.
There is a case where, substances which are rare in a biological tissue of a healthy portion are included in a great amount in a biological tissue of an affected area. Therefore, the spectral property of a biological tissue including an affected area differs from the spectral property of a biological tissue including only a health portion. As a result, by comparing the spectral property of a biological tissue with the spectral property of a known tissue not including an affected area, it becomes possible to make a diagnosis as to whether the biological tissue includes an affected area.